1. Field of the Invention
The present invention relates to a synchronous rectifier circuit.
2. Description of the Background Art
A DC-DC converter (a switching power supply) may suffer from heat generation by a forward voltage of a rectifier diode. In order to address this, a synchronous rectification DC-DC converter including a MOSFET (Metal-Oxide Semiconductor Field-Effect Transistor) low in ON resistance has been developed instead of the rectifier diode.
For example, Japanese Patent Laying-Open No. 2007-166794 discloses a technique for reducing electric power loss by achieving a faster OFF operation of a rectifier MOSFET. According to this technique, for driving the rectifier MOSFET, a third winding of a transformer, a rectifier circuit connected to the third winding, and an inductor and an impedance element connected to an output side thereof are provided. In turning off the rectifier MOSFET, energy stored in the inductor is released through a rectifier circuit. Consequently, charges in a gate capacitance of the rectifier MOSFET are rapidly released.
Japanese Patent Laying-Open No. 2002-335675 discloses a technique relating to a forward-type synchronous rectification DC-DC converter. The DC-DC converter described in this document includes a main switching element provided on a primary side of a transformer, respective switching elements for rectification and commutation provided on a secondary side, and first and second auxiliary windings for driving the switching elements for rectification and commutation in synchronization with switching on the primary side, respectively. Further, a transformer reset circuit for clamping a voltage waveform of an output from the second auxiliary winding is provided in parallel to the second auxiliary winding. The transformer reset circuit resets, during a commutation period in which the main switching element is in the OFF state, excitation energy accumulated in the transformer during a rectification period in which the main switching element is in the ON state.
Japanese Patent Laying-Open No. 2002-199713 discloses a technique for stopping self-oscillation of a synchronous rectifier circuit that often occurs when synchronous rectification switching power supplies are operated in parallel. A synchronous rectification type forward converter in this document includes a switch provided between a source of a rectifier MOSFET and a gate of a commutation MOSFET and a self-oscillation detection circuit of the synchronous rectifier circuit. The switch is controlled by a detection signal from the self-oscillation detection circuit and the source of the rectifier MOSFET and the gate of the commutation MOSFET are short-circuited or opened.
Synchronous rectification, however, suffers a problem of a flow-through current that flows when a MOSFET is switched with a time lag from desired timing. For example, in a forward type DC-DC converter, when a rectifier MOSFET and a commutation MOSFET are simultaneously turned on, a flow-through current flows from a high voltage side to a low voltage side. In a flyback type DC-DC converter, when a MOSFET on the primary side of a transformer is turned on while a rectifier MOSFET on the secondary side remains in the ON state, a flow-through current flows through the rectifier MOSFET because such a state is equivalent to a state where the circuit on the secondary side is short-circuited. Flow of the flow-through current may lead to breakdown of the MOSFET.